Lightning is caused by the build up of electrostatic charge in clouds. One region within the cloud builds up a positive charge and the other a negative charge. The process is not completely understood as to why, but the bottom of the cloud usually ends up being negatively charged and the top positively charged. If the build up (separation) of charge becomes great enough, the negative charges may leap to the positive side of another cloud, this is called sheet lightning or it may leap to the ground. The animation shows he creation of a ground strike of lightning.

As the negative charges collect at the bottom of the cloud it forces the negative charges in the ground to be forced away from the surface. This leaves the ground positive.

A streamer of negative charges is repelled by the bottom of the cloud and attracted by the ground.

As this streamer of negative charges approaches the ground, a streamer of positive charges is repelled by the ground and attracted to the negative streamer.

When the two streamers connect, they have created a fairly conductive path which allows a sudden down surge of electrons to jump to the ground. This is the lightning.

The rapidly moving electrons excite the air along the path so much that it emits light. It also heats the air so intensely that it rapidly expands creating thunder.

One thing to notice is that the positive charges that make up both the cloud and the ground do not move. Even the positive streamer launched by the ground is really only made up of positively charged air particles because the electron(s) left the particle.